The present invention relates to generally to a speed detector for use with a miniature DC motor, and more particularly to a speed detector of the type used for measuring the rotational speed of the miniature DC motor by detecting the magnitude and direction of leakage flux from the field pole assembly.
One type of the conventional speed detectors for use with miniature DC motors comprises, in general, a toothed wheel made of magnetic material and rigidly attached to the shaft of the DC motor. In addition, an exciting magnet and a magnetic detecting head are both mounted upon the frame of the DC motor. Upon rotation of the DC motor, the toothed wheel changes the magnetic path between the exciting magnet and the magnetic head so that an output signal having a frequency proportional to the rotational speed of the DC motor is derived from the output terminals of the magnetic head. The speed detector of the type described is, however, complex in construction, requires an additional space for installation of the speed detector outside of the DC motor, and consists of a relatively large number of parts, so that the productivity is low.
To overcome these inherent limitations, there has been proposed a speed detector of the type for detecting the rotational speed of the DC motor by detecting the variation of the leakage flux with a magnetic head. However, this device has an inherent deficiency in that the precise detection of the rotational speed cannot be made because the pulse-like noise produced when the armature current is interrupted and reversed in direction and the noise caused by the variation in contact resistance between the brushes and the commutator are both superimposed on the output signal.